Add additional trigger events to CI workflows

The workflow_dispatch and repository_dispatch events allow manual triggering of the workflow. These can be useful when
doing development and maintenance on the CI system, and do no harm when they are not needed, so it makes sense to add
them.

Author: per1234

.github/workflows/spell-check.yml

@@ -6,6 +6,8 @@ on:
   schedule:
     # run every Tuesday at 3 AM UTC
     - cron: "0 3 * * 2"
+  workflow_dispatch:
+  repository_dispatch:
 
 jobs:
   spellcheck: